The present invention relates to a carrier injection type light emitting semiconductor device utilizing a quantum effect.
A light emitting semiconductor laser with one or more quantum wells in its active region is referred to as a QW laser. A QW is formed by a semiconductor layer having a band-gap narrower than the surrounding material and a thickness smaller than the de Broglie wavelength. An injection type QW laser has a QW active region formed within the optical guide region of a semiconductor laser having a double-hetero (DH) structure. In such a QW laser, electronic motion is quantized perpendicular to the semiconductor layer. For this reason, the wave function is localized perpendicular to the semiconductor layer, and a two-dimensional electron gas (2 DEG), having freedom of movement only in a direction parallel to the semiconductor layer, is formed. The density of states of this 2 DEG rises abruptly from zero at the band edge. Therefore, the QW laser has a higher light emission efficiency than normal DH lasers. Other special features of the QW laser include its small threshold current, its capability of high-speed modulation, and the variability of its oscillation frequency. Because of these features, the QW laser is used in both optoelectronic IC (OEICs) and high-speed modulation optical device applications.
Nevertheless, in a conventional QW laser, an internal electric field exists in the active region under zero bias conditions, and this prevents any further improvement in QW laser characteristics.
FIG. 13 shows the energy band diagram and doping concentration profile of a conventional QW laser with an SCH (Separate Confinement Hetero) structure, i.e. a structure in which the carrier and optical confinement regions are distinct from each other. Reference numeral 11 denotes a p-type cladding layer; 12, an intrinsic optical wave guide layer; and 13, an n-type cladding layer. The p-type cladding layer 11 injects holes into the optical wave guide layer 12, and the n-type cladding layer 13 injects electrons into layer 12. Layer 12 has a semiconductor band gap narrower than those of layers 11 and 13, and makes optical confinement possible. Active region 14 is formed inside layer 12. Active region 14 commonly has a multi-quantum well (MQW) structure, in which a plurality of narrower band gap semiconductor layers (QW layers) are stacked so that a wider band gap semiconductor layer (barrier layer) is sandwiched between each two QW layers. For the sake of simplicity, FIG. 13 shows an energy band diagram with only one QW layer. This diagram corresponds to the zero bias state of the diode. As can be seen from FIG. 13, in this QW laser, there is an internal electric field in region 12 resulting from the difference in electron affinities of layers 11 and 13.
FIG. 14 shows the energy band diagram when a forward bias voltage Vb is applied to the above QW laser. When a forward bias voltage is applied, electrons and holes are injected into region 14 from layers 13 and 11, respectively. FIG. 14 shows a state wherein the bias voltage Vb is smaller than the diffusion potential Vbi. In practice, the QW laser begins to oscillate in this state, i.e., in a state in which there exists an internal electric field of about 10 kV/cm in active region 14. This causes the oscillation thereshold current of the QW laser to be higher than it need be. The reason for this is as follows.
Firstly, when an internal electric field exists in active region 14, the quantized electron wave function 15 and the quantized hole wave function 16 are displaced towards opposite edges of the QW. This causes a quenching of the optical transition matrix element. For the MQW structure, this quenching effect is large unless the resonant energy between the QW layers is very large.
Secondly, when an internal electric field exists in the active region, the potential of a barrier layer between two QW layers is decreased. For this reason, carriers injected into a QW layer can easily escape therefrom by tunneling, thereby degrading the desired carrier confinement. This is known as Fowler-Nordheim tunneling. Carriers escaping from the QW layer cause non-radiative recombination. As a result, the quantum efficiency of the QW laser is degraded.
For the above reasons, a conventional QW laser has a larger oscillation threshold current and a smaller quantum efficiency than is its inherent capability.